Rice Team Observes Limited Atoms in Bose-Einstein

CONTACT: Lia Unrau

PHONE: (713)
831-4793

E-MAIL: unrau@rice.edu

Randall Hulet

PHONE: (713) 527-6087

RICE TEAM OBSERVES LIMITED ATOMS IN
BOSE-EINSTEIN

Of the three teams in the world that can coax
the elusive Bose-Einstein condensation into existence, only the Rice University
team can make it using atoms that attract each other.

This provides a unique situation for studying the interactions of the atoms
in this rare state of matter. For this reason, the Rice team is taking an
especially close look at the mechanics of how their condensate forms and the
special properties it possesses. Their findings contribute to a basic
understanding of interactions on the atomic level.

A Bose-Einstein condensate, independently postulated by Indian physicist
Satyendranath Bose and Albert Einstein in the 1920s, is a rare state of matter
in which the atoms are so cold and move so slowly that they undergo a phase
transition, as when water freezes to ice. But in this phase transition, the
atoms condense into a special gas which behaves as a single unit. This occurs at
fractions of a degree above absolute zero, or minus 460 degrees Fahrenheit.

In the Feb. 10 issue of Physical Review Letters, the Rice team, led by
Randall Hulet, professor of physics, reports that when the condensate is formed
with attractive atoms, specifically lithium, there is a ceiling to the number of
atoms before the condensate maxes out and collapses.

Quantum theoreticians predicted that condensates with atoms that attract each
other are limited to about 1,400 atoms. The Rice group found a maximum between
650 and 1,300 atoms, which is consistent with the predictions.

The limit on the number of atoms is in contrast to condensates formed with
atoms that repel each other-there is no known limit to the number of atoms
possible in those, and as many as five million atoms have been reported.

Since the Rice group first indirectly observed a Bose-Einstein condensate in
1995, Hulet and graduate students Curtis Bradley and Charles “Cass” Sackett have
been working to improve their lab apparatus so they could directly see the
condensate peak and distribution of the atoms. When they first saw the state of
matter, they had to infer its existence from distortions it produced in a laser
beam passed through it.

Using technology borrowed from biologists, Hulet, Bradley and Sackett were
able to adapt what is called phase-contrast imaging to get their measurements.
Biologists use this technique to look at transparent objects such as cell
membranes. Light passing through the membrane is made to interfere with light
not passing through it, producing an image of the transparent material.

“We have developed a similar technique,” Hulet says, “that makes use of the
phase shift of the atoms. Now there’s no distortion, and we get a cleaner
picture of the condensate.”

Currently, the group is busy studying the interactions of the atoms in the
condensate-how it forms, how it begins to decay, and how it ultimately
collapses.

“What happens when it collapses is similar to the way a star collapses,” says
Hulet. “At some point, the attractive interactions overwhelm the `quantum
pressure’ that stabilizes the condensate and it implodes. We want to watch this
happening. This is an example of a macroscopic quantum mechanical process, which
is rare in physics.”

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